Introduction:
If you haven't seen our first two posts, I'd strongly recommend having a look at them as they talk in more detail about our plans and designs for the remote control plane, the building of which we'll focus on in this post.
Building Blocks:
T - 4 DAYS:
We commenced our first session by cutting out the rough shape of the fuselage. We drew on the balsa wood block with a pencil where we would saw, leaving ample space between our intended final size and the cuts to avoid cutting mistakes ruining our wood. We achieved this, and immediately started filing towards a more symmetrical, plane-like shape. Our aim was to have a nose in the cargo-plane style, which while fitting our look, had the far more useful purpose of allowing more space in the plane for components to fit - for this was our challenge, to fit most of our components, including a very bulky LiPo battery and two ESCs (surprisingly large), into the fuselage of our plane - maybe you’re starting to understand why we chose a cargo plane… Over the subsequent meetings, we refined the model to improve aerodynamics and symmetry, with no more major changes to the appearance of the fuselage.
Following this, we created an indent in the back of the fuselage for the 3D printed tail, which had a base shaped as a cuboid, as shown. We then drilled into the wood, which allowed us to insert bolts that would hold the tail section in place. At this point, we now had the plane shown below - it had a relatively realistic and aerodynamic fuselage, horizontal and vertical stabilisers, a rudder and one single elevator (as opposed to the two that full scale aeroplanes have) for simplicity of design. It’s worth noting at this point that we had been surprised by the low weight of the balsa wood - it felt almost negligible in your hands. This can be visualised through the maths - we had a 0.075 x 0.100 x 0.450m block of wood with density 150kg/m^3. As mass is density x volume, our weight would be acceleration due to gravity (g=9.81m/s^2) multiplied by density multiplied by volume. This gives:
g x (density x volume ) = weight
9.81 x [150 x (0.075 x 0.100 x 0.450)] = 4.97N
So the total weight of the wood was 5.0N, which translates to roughly half a kilogram of mass. By the time we had carried out the next step, the weight of the wood will have a significantly decreased further, as I will show.
T - 3 DAYS:
The next step was very much a make or break point in the building of our remote control plane. To store components inside the plane, we would have to find a way to make a hole in the centre of the plane while maintaining the shape and structural strength, and ensuring that components would be secure and accessible, for example to charge the battery. After discussion, we agreed that the easiest way to achieve this would be to saw the plane cleanly in half along the x axis, leaving an upper fuselage and a lower fuselage as shown. We executed this exactly as we intended, with the halved fuselage shown alongside the original block of wood.
Once this had been achieved, we began to chisel out a strip from the belly of the plane to store our components. Knowing that our LiPo battery was our largest component, we made the hold marginally larger than this. To ensure the fuselage was strong enough to support the components, we ensured that there was 1-2cm between the wall of the hold and the surface of the lower fuselage. This meant that we had to make a 2.5cm indent into the upper fuselage for the LiPo. A later addition was made to help our components sit comfortably in the belly, where we used a craft knife to cut grooves for our wires and connectors, leaving a cuboid shape to minimise volume loss to ill fitting components.
Now we can work out the total weight of the balsa wood, with the belly of the plane removed.
We can assume that the volume sawed and filed off to make a fuselage shape is around 40% of the original block, making our fuselage volume (not including volume lost to the cargo section) 0.0013m^3.
However, we chiselled out a block of wood of length 0.25m, width 0.035m, and depth 0.025m, giving a further volume loss of 0.00022m^3, combining with our original loss to give a final estimated plane volume of 0.0016m^3. Thus:
[ g x (density x volume of block )] - [ g x (density x volume of plane)] = weight
[9.81 x [150 x (0.075 x 0.100 x 0.450)]] - [9.81 x [ 150 x (1.56e-3) ]] = 2.66N
So the wood fuselage of our plane added a meagre 2.7N to our weight - less than 0.3kg! In terms of our final weight, this was all but negligible, meaning that choosing balsa wood was definitely the correct choice all things considered. However, it's not all good news; the low mass of our fuselage meant that the relatively heavy tail section (3D printed plastic can have a density of up to 1000kg/m^3 where our balsa wood was just 150) would have a larger effect on the positioning of our centre of mass.
And that's it for our third post in the RC Plane series! Just as before, if you have any questions about this, ask them in the comment section or drop us an email using the "contact us" page. Stay tuned for the next instalment, which we aim to post next week.
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